The present invention relates to methods for reclaiming metal catalyst species. In particular, this invention relates to methods for isolating and recycling palladium and cobalt catalysts used in the production of diaryl carbonates.
Diaryl carbonates, and diphenyl carbonate in particular, are valuable monomer precursors for the preparation of polycarbonates by melt transesterification. An advantageous route for the synthesis of diaryl carbonates is the direct carbonylation of aromatic hydroxy compounds by carbon monoxide and an oxidant in the presence of a catalyst.
A wide range of catalysts may be used in this preparation of diaryl carbonates. For example, U.S. Pat. No. 4,187,242 to Chalk discloses catalysts derived from Group VIIIB metals, i.e., metals selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or complexes thereof. U.S. Pat. Nos. 5,231,210 to Joyce, et al., U.S. Pat. Nos. 5,284,964 and 5,760,272 to Pressman et al., and 5,399,734 to King, Jr., et al. further disclose the use of co-catalysts, including metal co-catalyst species such as cobalt pentadentate complexes and complexes of cobalt with pyridines, bipyridines, terpyridines, quinolines, isoquinolines, aliphatic polyamines such as ethylenediamine, crown ethers, aromatic or aliphatic amine ethers such as cryptanes, and Schiff bases, in combination with organic co-catalysts such as terpyridines and quaternary ammonium or phosphonium halides. In U.S. Pat. No. 5,498,789 to Takagi et al., the catalyst system consists of a palladium compound, at least one lead compound, at least one halide selected from quaternary ammonium halides and quaternary phosphonium halides, and optionally at least one copper compound.
As can be seen from the above brief review, the crude reaction mixtures arising from the production of diaryl carbonates can contain complex mixtures of catalyst and co-catalyst metals, and organic products and by-products. The cost of commercially implementing direct oxidative carbonylation is heavily dependent on a combination of the efficiency of the catalyst package and on the ability to reclaim the expensive catalyst components and unconverted aromatic starting material. While palladium represents the primary material expense, it is also important to control the usage of other, less expensive materials, such as cobalt, manganese, or lead both from the cost and environmental points of view. Accordingly, there remains a need in the art for efficient, convenient methods for the reclamation of the metal catalysts and cocatalysts used in the carbonylation of aromatic hydroxy compounds to produce diary carbonates.